Abstract:

The plateau edge on the north bank of Baoji-Fufeng section in the middle reach of Weihe River is the concentrated development zone with large loess landslide, which consists of 58 places each with the area above 10 million cubic meters. Based on the detailed survey on the regional active fractures, geophysical survey, and detailed survey on typical slide structures, the authors studied the slope structure and mechanisms of landslides along the plateau edge. The study shows that: 1)Active faults in northern margin of the Weihe River, as graben-like structure, control the terrain and physiognomy of slope zone of the plateau edge on the northern bank. The details are that principal fracture surface is S-dipping and the dipping angle is 68° on average controls the angle of plateau edge in bank slope; and the assemblage of secondary faulted structure controls the slope structure and affects the structure of large loess landslide on the plateau edge. 2)The principal fracture surface of the plateau edge is consistent with smooth wall of back scarp of many large loess landslide, and it has influence on the configuration and occurrence of large landslide. The maximum distance of fault offset is 7.1 m. 3)Active faults have graben-like structure and local secondary fracture surface controls the structure of multi-order sliding surface of plateau edge and its formation and evolution. For instance, in the Yangjiacun landslide, local landslide is controlled and sheared by faulted structure and the shearing angle reaches 72°. The toe of surface of rupture of most landslides are not affected by fracture surface and their angles are 23° on average. 4)The clay content of clayey layer in the Sanmen Formation overpasses 35% and the average plastic index is 23. It is an interlayer shear zone of landslide. 5)The major pattern of new landslide development is reactivation of old ones induced by the precipitation. Many old landslides on northern bank were formed in the denudation period of second-order terrace in 0.009 Ma BP, while ancient landslides in the period of third-order terrace in 0.13 Ma BP.

Key words: slope, structure, fracture, landslides, mechanisms